yanniru: My reference is Born & Wolf, Principles of Optics. In there you will find that when light propagates in materials, its group velocity slows down. Ther are so-called precursors that propagate through the materials at the speed of light, but precursors can carry no information. Greg: When you are done patting yourself on the back, try reading what you wrote, then explain what speed you are talking about when you say the precursors propagate through the materials at the speed of light. Is this some different speed of light?